Please post your System Information as Lightroom Classic (LrC) reports it. In LrC click on Help, then System Info, then Copy. Paste that information into a reply. Please present all information from first line down to and including Plug-in Info. Info after Plug-in info can be cut as that is just so much dead space to us non-Techs.
Now I use LR but LR does not seem to find the NAS.
So my question is: do I have to upload a plug-in to 'connect' my NAS with LR?
Lightroom Classic does not "find" photos or drives, you have to tell Lightroom Classic that the photos are there. The primary way is by importing photos from a drive (internal, external, NAS) and only then does Lightroom Classic know about the drive.
Have you done that? If you have, then I don't understand what you mean by "find" in the context of Lightroom Classic. If you have not done that, then that's the first step.